1. Field of the Invention
The present invention generally relates to digital communications and, more particularly, to asynchronous communications between two or more computers wherein the computers are continuously connected by a physical link but operate autonomously between communications. The invention has particular application to computer controlled manufacturing processes and tools. In a process control environment, a main computer coordinates a series of tasks by interfacing with various types of peripheral equipment. These peripherals can be inputs (e.g., pressure or proximity sensors) or outputs (e.g., actuators or motors) controlled directly by the main computer or indirectly by a slave computer. In the case of a distributed system where a main computer communicates asynchronously with one or more slave computers, the most common interface is the RS-232-C serial input/output (I/O) standard.
2. Description of the Prior Art
There are a number of standards organizations which are important to the establishment of industry standards for digital communications. Among these are the Electronics Industries Association (EIA) and the International Standards Organization (ISO). The EIA publishes standards such as RS-232-C that govern the electrical characteristics of connections between computers and various peripherals. The RS-232 connection is a standard serial input/output (I/O) connection, and the connector normally used for this connection is the DB-25 connector, the design of which is controlled by ISO document 2113.
Within the RS-232 standard, there exist two types of communication; half-duplex and full-duplex. In half-duplex communication, blocks of data can be transmitted in both directions, but in only one direction at a time. Traditional hardware handshaking for half-duplex communication between a DTE (data terminal equipment) and a DCE (data communications equipment) is as follows:
1. The DTE processor initiates contact by raising the data terminal ready (DTR) line.
2. The DCE processor responds by raising the data set ready (DSR) line.
3. The DTE processor continues by raising the request to send (RTS) line.
4. The DCE processor responds by raising the clear to send (CTS) line.
5. Data transmission from the DTE to DCE begins. The foregoing protocol is an example of hardware "handshaking". In addition to the hardware handshaking, various protocols can be incorporated in the data string to provide dam integrity. Examples are parity, check sum, byte count and framing characters.
In full-duplex communication, blocks of data can be transmitted in both directions at the same time. These two-way channels do not require RTS/CTS handshaking leading to a more efficient interface. The disadvantage is the complexity of frequency division multiplexing (FDM) required to prevent interference.
While the RS-232 handshaking protocol, which is an industry standard, works well for most computer communications, a problem has been encountered in applying this protocol to a computer controlled manufacturing environment. More specifically, a manufacturing tool which performs a complex series of operations can be effectively controlled by a plurality of micro computers, thereby dividing the tool into a plurality of less complex "subtools" each controlled by one of the micro computers. The micro computers, however, typically do not have sufficient capacity to perform more than a limited number of tasks and, in any case, must be coordinated so that the sequence of operations is performed as designed. The task of coordinating the several micro computers is usually assigned to a main computer.
The main computer thus controls the tool sequencing, while the micro computers are responsible for specific tasks, such as motion control. In this configuration, there are two types of communication; command communication and status communication. When the main computer communicates a task or command to a micro computer in order to begin a tool operation, the main computer is considered the master and the micro computer is the slave. On the other hand, when the micro computer has finished the task or command, it initiates communication with the main computer to update its progress. In this case, the micro computer is considered the master and the main computer is the slave.
Between communications, the main computer and the micro computers operate autonomously, even though they remain physically connected via a cable link. From this example, it can be seen that an individual computer will alternate from a master to a slave. This state change of each computer precludes the use of the EIA industry standard hardware RS-232 handshaking protocol.